Photoelectric sensors use light to sense targets without physical contact and are used in a wide variety of applications and environments, such as to sequentially detect the presence of objects on a conveyor belt, or to detect a change in the size, shape, reflectivity, or color of a target. Various types of photoelectric sensors are available, including transmitted beam sensors, retro-reflective sensors, and diffuse sensors. Each of these sensors includes a light source, such as a light emitting diode (LED) or a laser, and a photodetector for detecting light, such as a photodiode or a phototransistor, and can also include one or more lenses to focus the light emitted by the light source and/or to focus the received light for efficient detection by the photodetector. These sensors typically also include output circuitry in communication with the photodetector for producing a voltage or a current signal indicative of a characteristic of the sensed target, such as a high or low state for indicating the presence of a target at a predetermined location.
A transmitted beam type photoelectric sensor is arranged such that the light source is located on one side of a path of a target to be sensed, and the photodetector is located on the other side of the path. A light beam from the light source is directed to the photodetector, and when the target is absent, the light from the light source is detected at the photodetector. When the target is at a predetermined location, it partially or completely blocks this light beam from being received by the photodetector. A resulting change in the amount of light detected by the photodetector gives rise to an output signal indicative of the presence of the target.
As for the retroreflective and diffuse type sensors, both of these sensor types combine a light source and a photodetector in a single housing. A retroreflective type sensor uses a reflector located on an opposite side of a path of a target to be sensed, and this reflector reflects a light beam from the light source back to the photodetector. The presence of the target partially or completely blocks this light beam from being received by the photodetector. As shown in FIGS. 1(a) and 1(b) (prior art), a diffuse type sensor 10 operates by directing a light beam 16 towards a predetermined location on a light path and using a target 20 itself, when at the predetermined location (as shown particularly in FIG. 1(b)), to reflect a portion of the light beam 16 from the light source 12 back to the photodetector 14, which then detects more light than when the target is not at the predetermined location (as shown particularly in FIG. 1(a)). In particular, when the target 20 is present at the predetermined location, the light beam 16 strikes the target 20 at some arbitrary angle and is detected when the photodetector captures some portion of the reflected diffused light 18. Diffuse type sensors are well suited for applications with space requirements that limit the positioning of a reflector across from the photodetector.
Successful sensing requires that a change in the position, size, shape, color, or reflectivity of the target causes a sufficient measurable change in the amount or intensity of light detected by the photodetector. The performance of a photoelectric sensor detecting the presence and absence of a target can be quantified using the concept of margin or excess gain. Margin is a measurement of the amount of light from the light source that is detected by the photodetector compared to a minimum light level required to switch the output signal of the sensor (such as from a level indicative of the absence of a target to a level indicative of the presence of a target), and can also take into account a sensitivity of an output circuit. A margin value corresponds to a specific sensing distance between a target to be sensed and the sensor. A margin of zero occurs when none of the light emitted by the light source can be detected by the photodetector, and a margin of one occurs when just enough light is detected to cause the output signal of the sensor to change states. A margin of twenty (commonly expressed as 20×) can occur when twenty times the minimum light level required to switch the output signal of the sensor is detected by the photodetector, or can occur for example when 4 times the minimum light level is detected and the sensitivity of an output circuit is 5 times the sensitivity at the minimum light level. The higher the margin, the more capable a photoelectric sensor is at sensing a target at that distance.
Margin is measured and expressed relative to the reflectivity of the reflecting surface, for example relative to a white paper having a reflecting surface rated at 90% reflective, which will reflect more light and therefore allow for a larger margin than a paper surface that is 18% reflective. Typical margin response curves are often provided for a photoelectric sensor and show what the typical margin will be depending on the sensing distance (the sensing distance for a diffuse type sensor is defined as the distance from the sensor to the specified target).
Photoelectric sensors are often characterized in terms of their maximum and minimum sensing distances. For diffuse type sensors, often a “blind area” exists in which a target that is too close to the sensor cannot be sensed because the light reflected from the target cannot be received by the photodetector. For example, with a sensor designed to operate with a target at a sensing distance of up to 800 mm, this blind area can extend from zero to 50 mm. Referring again to FIG. 1(b), in the case of a diffuse sensor, this occurs because the light source and the photodetector are not coaxial so the light 16 emitted by the sensor and the reflected light 18 detectable by the photodetector travel along different paths, with the reflected light 18 typically entering the sensor 10 at an angle with respect to the emitted light. When the target 20 becomes too close to the sensor 10, none of the reflected light can be detected by the photodetector, as the angle between the emitted light and the reflected light becomes too great.
It is desirable that a photoelectric sensor for sensing the presence of a target be operable over a target sensing range that encompasses both a far distance, such as 800 mm, and also a near distance, which preferably extends to zero or as close to zero as possible. With a conventional photoelectric sensor, for a target at a far distance, in order to obtain a sufficient measurable change in the light detected by the photodetector from the target, it may be necessary to increase the intensity of the emitted light, and/or increase the sensitivity of the photodetector. Each of these modifications results in an increased margin corresponding to that far distance, but there are limits on these modifications. Generally, the emit power is limited by characteristics of the light source such as expected life and maximum current, as well as safety considerations in the case of a laser light source. Further, one drawback to increasing the sensitivity is that more EMI (electromagnetic interference) signals (noise) can also be detected by the photodetector, resulting in an inaccurate measurement for a target to be sensed. This can effectively restrict the sensitivity of the photodetector.
Further with a conventional photoelectric sensor, increasing the diameter of a receive lens that directs light to the photodetector can also result in an increased margin by allowing more light to be detected by the photodetector. However, such increases also increase the minimum sensing distance, that is, they limit the photoelectric sensor's ability to sense targets located close to the photoelectric sensor. Referring again to FIG. 1(b), in the case of a diffuse sensor, this occurs because the light source and the photodetector are not coaxial so the light 16 emitted by the sensor and the reflected light 18 detectable by the photodetector travel along different paths, with the reflected light 18 typically entering the sensor 10 at an angle with respect to the emitted light. When the target 20 becomes too close to the sensor 10, none of the reflected light can be detected by the photodetector, as the angle between the emitted light and the reflected light becomes too great. This is especially problematic in the case of a sensor having a laser light source, because the laser emits a narrower beam of light in contrast to the light emitted by a LED. Further, increasing the size of the receive lens also means that more environmental ambient light (light not originating from the light source) can be detected by the photodetector, resulting in an inaccurate measurement for a target to be sensed. With a photoelectric sensor, the light source typically emits pulsed light, so the light signal which is reflected emitted light which is received by the photodetector is also pulsed. A larger receive lens can collect more ambient light which makes it possible that the photodetector can become saturated if the ambient light is strong enough. A saturated photodetector means that the output signal from the photodetector is a constant DC current or voltage, so the received emitted light signal cannot be detected, resulting in another type of “blindness”. This can effectively restrict the size of the receiver lens.
For at least these reasons, it would be advantageous if an improved photoelectric sensor could be developed that overcame one or more of the above disadvantages. It would be further advantageous in particular if, in at least some such embodiments, the improved photoelectric sensor had a sufficient margin over a sensing range that included regions very close to (and possibly right up to) the sensor. It would also be advantageous if, in at least some such embodiments, the improved sensor could operate in the presence of ambient light, such as up to at least 5,000 lux, and preferably to 50,000 lux, and was relatively unaffected by EMI over its sensing range, especially with a sensor using a laser light source.